Is this save running RAC without Multipath ??

Question

Dear All,
I have been develop RAC 10g for Itanium
Proc : Itanium
OS : RHEL 5
Oracle : 10.2.0.5
Storage : HP eva 4000
for ocr and voting disk, i'm using OCFS2
for data, i'm using ASM with ASMLib
but, i don't use Multipath for the storage
when use fdisk -l, the disk is appear 4 times
here the example :
     sda     = sdh = sdo = sdv
My Question :
is this save running RAC without Multipath ??
( i'm sorry for bad english )
regards

Answer 1

The short answer is this: If you have multipath devices available to you, you should configure them properly to reduce points of failure.
The purpose of multipathing is to eliminate a single point of failure in the Disk/SAN connectivity. For example, if you have 2 host bus adapters (HBAs) in the server and 2 SAN processors (heads), you may have the SAN fabric cross-connected via one or more fiber switches so that if there is one physical connect lost, there is another physical connection that can route IO requests to the same LUN on the SAN. However, those multiple paths may look like different devices to the Operating system, which is where multipath comes in. If the OS or applications are unaware that the 4 mounted devices are actually one single LUN, there will not be an automated failover available, and manual reconfiguration to use a different device would be terribly time-consuming, and require downtime. For example:
Let's assume you have 4 LUNs mounted in a 4-path configuration, for a total of 16 devices
path1:
sd1, sd2, sd3, sd4
path2:
sd5, sd6, sd7, sd8
path3:
sd9, sd10, sd11, sd12
path 4:
sd13, sd14, sd15, sd16
Think about when you add these devices to an ASM diskgroup - what happens? You cannot select both sd1 and sd5, since they are the same LUN, and once ONE of the devices is added, the ASM header will be written to the device and it will not be flagged as a candidate any longer. So your ASM diskgroup would consist of 4 devices.
Now, what if sd1 fails? The OS and ASM are unaware that sd5 is the same LUN accessible via a different path, so if the path with sd1 failed, you would experience an extended downtime. Or, as another example, what if your storage administrator needs to do maintenance that affects one of the paths (i.e., firmware upgrade on a SAN head)? You would need to shutdown your database instead of working with the SAN administrator to do the upgrades in a rolling manner.
That's where multipathing's benefit is - if you aggregate sd1, sd5, sd9, sd13 with multipathing and create a device named (for example) "mp1" and repeat for each of the other multipath LUNs, then you would create your ASM diskgroup as "mp1, mp2, mp3, mp4". A failure on sd1 (in mp1) would simply be re-routed to sd5 or sd9 or sd13, with no impact to your diskgroup or ASM availability.
K